1. Field of the Invention
The current invention generally relates to optical lithography, and more specifically relates to optical lithography implemented in an immersion liquid environment.
2. Description of the Related Art
The widespread utilization of computer systems to solve complex problems has created a demand for applications designed to produce solutions using increasingly complex algorithms. As the complexity of the problems has increased, so too have the computational requirements of the applications used to solve these problems. The ability of computer systems to produce accurate results in an efficient manner is determined by both the application design and computer system hardware running the application.
Increases in computer hardware performance are continuously strained by design specifications that push the physical properties of the materials that comprise a computer chip. Increased demands in performance require computer chips with more wires on more layers to perform complex computations in acceptable time frames. The number of components and wires on a computer chip required to satisfy these demands has continually increased forcing computer chip designers to create denser chip layouts or increase the physical dimensions of the chip. Chip manufacturers have chosen to keep chip size relatively constant over time and increase the density of components on chips.
As wire, component, and layer density increased on computer chips, manufacturers struggled to maintain the necessary precision for useful operation. To meet increasingly demanding precision requirements, the method of optical lithography was implemented using light to print device and wire patterns on the chip layers. Optical lithography uses a light projection device including a lens system to concentrate light of a particular wavelength onto a wafer. The wafer is first coated with a resist material sensitive to light exposure. As the light scans the wafer, the image is printed onto the wafer. Alternatively, “scanning” may be referred to as “imaging” and used interchangeably within the field of optical lithography. The wafer is then chemically bathed removing any positive acting photoresist material exposed to the light scan. In the early 1990's, optical lithography was capable of producing linewidths of 0.35 micrometers in manufacturing, and current optical lithography techniques can produce linewidths of 100 to 150 nanometers in manufacturing. Smaller lines can be produced for developmental and prototype purposes.
The foundation of optical lithography is based heavily on Rayleigh's two equations. These equations define the dependency of resolution (W) and depth of focus (DOF) on wavelength and the numerical aperture (NA) of the imaging system where numerical aperture is defined as a measure of light gathering power of a lens {Lin, B. J. “New λ/NA scaling equations for resolution and depth-of-focus.” Optical Microlithography XIII (2000): 759.}. The resolution of imaging is defined in the Rayleigh equation: W=k1·λv/NA. Resolution is the minimum feature that may be printed using optical lithography and determines the fidelity of the pattern transfer. Depth of focus can be defined as the region around the image plane in which the image will appear to be sharp. (“Depth of field and depth of focus”, 25 Jul. 2000 at URL http://www.matter.org.uk/tem/depth_of_field.htm.) Based on Rayleigh's work, depth of focus is defined as: DOF=k2·λv/NA2 as derived for the paraxial case, where λv is a wavelength in a vacuum and NA=n sin θ where n is the index of refraction and θ is the acceptance angle of the lens.
For consistency in the high NA immersion case, Bum Lin has defined resolution as W=k1·λ/sin θ where λ=λv/n. Burn Lin has also shown for immersion optical lithography that DOF=k3·λ/sin2(θ/2), where λ=(λv/n), k3 is an engineering constant specific to the lithographic process, θ is the angle used to define NA, and λ is the wavelength (λv/n) in the immersion media. This second form is less ambiguous for high NA and immersion optical lithography.
Optical lithography has been extended to use 193 nanometers for manufacturing patterns, but problems begin to occur below this wavelength. As components and wire dimensions become smaller, the difference in size between the wavelength of the light and the components shrinks. The components and wires at some critical point become the same size or smaller than the wavelength of the light. At this point, the implemented wavelength is no longer capable of printing the chip design with sufficient fidelity. To overcome this problem, shorter light wavelengths must be used; however, new problems arise when using shorter wavelengths. Shorter wavelengths, such as x-rays have been used to achieve smaller linewidths, but the adoption of equipment capable of producing x-rays has been hindered by difficulties associated with manufacturing lenses capable of producing sufficient imaging quality when used with x-rays. These difficulties have led to high lens costs resulting in an expensive migration path from past optical lithography equipment to x-ray optical lithography equipment. Shorter wavelengths are also higher energy wavelengths and therefore high doses of x-rays have a greater potential to damage the solid chip material, especially dielectric. Furthermore, light sensitive compounds in resist only absorb light over a specific range of wavelengths and alternative materials may not always perform as well as necessary. See “Optical Lithography”, Craig Friedrich, 1998 at URL http://www.me.mtu.edu/˜microweb/chap1/ch1-4-1.htm.
One way to improve the resolution of optical lithography is to manipulate the numerical aperture variable in Rayleigh's equation or sin θ/2 in Burn Lin's equations. The maximum attainable value for numerical aperture in conventional dry optical lithography methods is 1; however, it is known from optical microscopy and the work of E. Abbe (1878) that by filling the space between the final lens and the wafer with a high index liquid, light that would otherwise be totally internally reflected is able to pass through the liquid to the wafer surface {Switkes, M., M. Rothschild “Resolution Enhancement of 157 nm Lithography by Liquid Immersion.” Optical Microlithography XV (2002): 459.}. It is possible to achieve numerical apertures greater than one and as high as the index of the immersion liquid. The use of a liquid in optical lithography increases the depth of focus by a factor equal to the index of the immersion liquid when NA is held constant, therefore increasing the tolerable error in the process.
Immersion optical lithography permits optical lithography exposure equipment manufacturers to extend the use of their current optical lithography equipment to the next generation of chip design with minimal development cost. With potential numerical apertures of 1.25 or higher and resolutions of 50 nanometers, future chips can be produced using modern immersion optical lithography techniques without making high risk, expensive expenditures on new capital equipment and resist materials required for shorter wavelengths. Because the properties of water make it an ideal immersion liquid for 193 nanometer imaging, and relatively minor modifications to existing equipment are necessary, the transition from dry optical lithography to immersion optical lithography is an economically feasible and low risk decision. New sources of light and new resists are also unnecessary.
The advent of immersion optical lithography has also resulted in numerous additional problems. In order to achieve maximum gains in numerical aperture size, there can be no air between the final lens and the immersion liquid. This requires the final lens element to be immersed in the liquid. Throughout the process, the wafer is secured to a horizontal support surface capable of moving in the x,y, and z directions. During scanning, the final lens element and/or horizontal support surface are moved as the wafer surface is scanned. As the lens moves through the liquid, the motion of the lens translates energy from the lens into the liquid, thus creating ripples, turbulence, and disruption of the liquid environment. Gas and air bubbles may become trapped within the liquid or attach to the lens surface resulting in light scattering and poor quality imaging. Therefore, a need exists for a device capable of minimizing the ripples and turbulences associated with the energy transfer between the motion of the lens and the liquid environment.